1. Field of the Invention
Embodiments of the present invention relate generally to optical communication systems and, more particularly, to a method of determining optical distance for chromatic dispersion compensation.
2. Description of the Related Art
In a wavelength division multiplexing (WDM) optical communication system, information is carried by multiple channels, each channel having a unique wavelength. WDM allows transmission of data from different sources over the same fiber optic link simultaneously, since each data source is assigned a dedicated wavelength channel. The result is an optical communication link with an aggregate bandwidth that increases with the number of wavelengths, or channels, incorporated into the WDM signal. In this way, WDM technology maximizes the use of an available fiber optic infrastructure-what would normally require multiple optical links instead requires only one. As defined herein, an “optical link” refers to an optical communications link established between two nodes in an optical network. An optical link may include a combination of optical fibers or other waveguides, free-space optics, and optical router or other switching apparatus.
When a light wave travels through a medium, such as an optical fiber, each wavelength contained in the light wave travels at a different speed, resulting in chromatic dispersion. Chromatic dispersion leads to signal degradation since the varying delay in arrival time between the different constituent components of a WDM signal causes the different wavelength channels of a WDM signal to arrive at a given destination node at different times. Thus, a misalignment in time is produced between the different wavelength channels of a WDM signal when transmitted from node to node within a system. In addition, chromatic dispersion “smears out” each wavelength channel over time, producing a phenomenon referred to as pulse spreading, i.e., broadening or distorting the profile of each optical pulse over time. Pulse spreading, if uncorrected, typically causes interference between adjacent light pulses, resulting in increased bit rate error.
To date, chromatic dispersion is generally addressed with fixed dispersion compensation. With such an approach, when an optical link between two nodes of an optical network is initially established, the chromatic dispersion of a WDM signal is measured, and an appropriate compensation module is installed in the optical link, such as a dispersion-compensating fiber (DCF). A DCF is an optical fiber connected in series with the signal-transmitting fiber of a link. The DCF has a chromatic dispersion slope opposite in sign to that of the signal-transmitting fiber, which reduces the absolute value of dispersion produced in the optical link.
One drawback to using fixed dispersion compensation is that it is unable to adequately correct for chromatic dispersion in evolving optical networks. For example, as bandwidth requirements for optical communication networks increase, it is desirable to increase the amount of information carried by a single optical fiber. This may be accomplished by increasing the transmission speed, or bit rate, within such networks. As the speed of WDM systems increases beyond 10 Gbps, the magnitude of the pulse spreading and the time offsets between wavelength channels caused by chromatic dispersion can approach the same time scale as the bit rate of the optical system. Because the chromatic dispersion is oftentimes a function of variable environmental conditions, an initial representative measurement of dispersion produced within the optical link may not be able to provide an accurate estimate of chromatic dispersion, especially with the accuracy required by high-speed optical systems.
In addition, the topography of optical networks is too complex for fixed dispersion compensation, as configurable, or dynamic, networks become more common. In configurable optical networks, the optical path from one transmission point to another does not remain constant. Instead, the optical path associated with a given transmission within a network may vary greatly since the network may be reconfigured in response to network utilization and other factors. Because dispersion is proportional to the optical distance traveled by a light wave, any dispersion estimate based on a substantially different optical path than that actually traveled by a light wave is of little practical use for dispersion compensation. For this reason, fixed dispersion compensation has limited utility in configurable networks.
To address these problems, tunable dispersion compensation (TDC) has been developed, which is a process that realigns each wavelength channel in time with the other wavelength channels making up a common WDM signal. Similar to fixed dispersion compensation, TDC is performed at a node in an optical network. Unlike fixed compensation, though, with TDC, the amount of dispersion compensation applied to the different wavelength channels of a WDM signal may be varied. For example, when environmental conditions in the optical link change or when the optical transmission is routed along a different optical path within the system, i.e., along a different combination of waveguides and routers, more or less dispersion compensation may be applied to the different wavelength channels of the WDM signal. For this reason, tunable dispersion compensation is preferred over fixed dispersion compensation for configurable networks.
One problem with TDC, though, is that proper compensation of chromatic dispersion in an optical link requires accurate knowledge of the chromatic dispersion produced in the link. Theoretically calculating all factors affecting chromatic dispersion is impracticable and, hence, unreliable for predicting dispersion in a given link. This is because the refractive index of an optical medium is a function not only of the material making up the optical medium, but also of internal mechanical stresses on the optical medium. Examples of refractive index-altering factors include stresses induced in optical components during installation of the optical network and stresses caused by thermal expansion and contraction of such components, all of which are difficult to quantify. For this reason, current TDC techniques rely on directly measuring dispersion in the optical link. Measuring chromatic dispersion typically involves estimating propagation losses based on pulse transmissions between optical network nodes. For a variety of well-known reasons, this approach is inherently inaccurate and impractical for use in conjunction with TDC. For example, a one-time estimate of the chromatic dispersion ignores subsequent changes in the dispersion, including changes caused by environmental factors, such as thermally-induced stresses in optic fibers, and changes that occur whenever an optical network is reconfigured. In addition, estimating the optical length of a link based on propagation losses does not take into account factors that increase propagation loss but do not add optical length, such as the presence of fiber splices or connectors.
Accordingly, there is a need in the art for a more accurate technique for measuring optical distances for purposes of tunable chromatic dispersion compensation.